sendmail address rewriting mini-HOWTO
Thomas Roessler, roessler@guug.de
v0.0, 6 May 1998
This document is a brief description of how to set up sendmail's con­
figuration file for the home user's dial-up access.
1. Introduction
We assume that you have the kind of Internet access which seems to be
most common at universities and online services nowadays: You dial
into your provider's network using PPP over a serial connection. Your
incoming mail is spooled at the provider's POP or IMAP server, while
outgoing messages are to be sent via SMTP. You don't have a domain
name of your own, so everything has to use one address.
We assume that you have already installed a fairly recent version of
Eric Allman's sendmail (version 8.8.8 is current at the time of this
writing and should work fine).
This document is partially referring to specific properties of Debian
GNU/Linux systems; users of different distributions will have to take
some care.
Make sure you have the following information at hand:
· Your ISP's mail server
· Your Internet mail address
The configuration we are planning has two main goals:
1. Sending mail between various local users must be possible.
2. The outside world must see the local users' ISP mail addresses, not
the local ones.
To achieve this, we will make use of sendmail's genericstable feature.
2. File Roadmap
We will put all of sendmail's configuration files in a separate
directory under /etc: /etc/mail. Usually, sendmail will expect these
files to reside directly under /etc. To avoid problems,
/etc/sendmail.cf should be a symbolic link to /etc/mail/sendmail.cf.
The following files will populate /etc/mail:
· =20
· aliases - contains additional local addresses
· genericsdomain - contains some information on your local host's
configuration
· genericstable - contains the actual rewriting rules.
· sendmail.cf - sendmail's configuration file
· sendmail.mc - the source of sendmail.cf.
Some of these files will be accompanied by .db files. They contain
hashed databases for sendmail's direct use.
We assume that the cf part of sendmail's source tree resides under a
directory named /usr/lib/sendmail.cf. This is the case on Debian
GNU/Linux systems. Other distributions will put this stuff at
different places. Please refer to your distribution's documentation
for details.
3. Configuring sendmail
3.1. The main configuration file
Sendmail uses a highly complex rule system for it's configuration.
While you can do lots of neat tricks with this stuff, writing a
sendmail.cf file from scratch is rather unusual and time-consuming.
If you are interested in doing so, you should stop reading this
document right now and instead read the "Bat Book" from O'Reilly.
Instead of hand-crafting these rules, we will rely on the m4 macro
processor to put together our configuration file from ready-made
pieces which are distributed together with sendmail.
Let's look at the first lines of the sendmail.mc file:
______________________________________________________________________
include(/usr/lib/sendmail.cf/m4/cf.m4)
VERSIONID(`sendmail.mc - roessler@guug.de')
OSTYPE(debian)
define(`ALIAS_FILE',`/etc/mail/aliases')
______________________________________________________________________
In the beginning, cf.m4 is included. This m4 macro file contains lots
of macro definitions for the rest of the file. Be sure that the path
you give here is correct - the one we are representing in our example
is typical for Debian GNU/Linux. The OSTYPE macro is used to give
some useful defaults for certain configuration values. If you aren't
using a Debian system, you should replace the word "debian" by "linux"
here. ALIAS_FILE tells sendmail where to look for the list of
aliases.
The following lines tell sendmail to use the genericstable feature,
and where to find the configuration files needed to use it:
______________________________________________________________________
FEATURE(masquerade_envelope) FEATURE(genericstable, `hash
-o /etc/mail/genericstable')
GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain')
______________________________________________________________________
The masquerade_envelope feature tells sendmail to apply header rewrit­
ing to the envelope sender of a message. This is the mail address to
which external mail delivery subsystems will direct their delivery
failure reports and warning messages. The generics* files will be
explained below.
Now, we have to define a so-called smart host, that is, a machine
which will handle outgoing mail for your system. Note that this
machine may be different from your ISP's POP and IMAP servers. If in
doubt, contact the hotline. The code in the master configuration
file:
______________________________________________________________________
define(`SMART_HOST',`mail-out.your.provider')
______________________________________________________________________
Please replace mail-out.your.provider by the fully qualified hostname
of your internet service provider.
The final two lines include the "mailer" definitions which are needed
by sendmail to find out how to handle various types of mail:
______________________________________________________________________
MAILER(local)
MAILER(smtp)
______________________________________________________________________
To generate the sendmail.cf file from this sendmail.mc, type the
following commands (as root):
______________________________________________________________________
# m4 sendmail.mc > _sendmail.cf
# mv -f _sendmail.cf sendmail.cf
______________________________________________________________________
Note the technique of writing m4's output to a temporary file which is
thereafter moved to the proper place. This helps us to prevent send­
mail from reading partially written configuration files.
3.2. Address rewriting
First, we have to tell sendmail what addresses are to be considered
local (and thus should be subjected to the rewriting). This is quite
simple: Just put the fully qualified host name of your machine into
the file /etc/mail/genericsdomain. To get your host's fully qualified
name, type the following command:
______________________________________________________________________
$ hostname -f
______________________________________________________________________
Now, let's come to the rewriting table proper:
/etc/mail/genericstable. This file consists of two white-space
separated columns. The first column contains the local address, the
second column contains the e-mail address which should be used
instead. The file may look like this:
______________________________________________________________________
harry harryx@your.isp
maude maudey@her.isp
root fredx@your.isp
news fredx@your.isp
______________________________________________________________________
Note that there should be one entry for each account on the local
machine, so that automatically generated mail which leaks out of the
local system carries correct header information.
For performance reasons, sendmail won't use this text file directly,
but rely on a "hashed" version instead. To generate it, type the
following command:
______________________________________________________________________
# makemap -r hash genericstable.db < genericstable
______________________________________________________________________
Note that the rewriting rules from the genericstable will not apply to
local mail or to messages you receive from outside - the mapping is
only used if a message leaves your local system for your ISP's smart
host.
3.3. Aliases
The aliases file contains additional local names which are only valid
for local messages. This is useful for administrative accounts like
root which receive automatically generated messages from your system.
A reasonable start for /etc/mail/aliases could look like the following
file:
______________________________________________________________________
root: fred
news: root
postmaster: root
mail: root
www: root
nobody: /dev/null
MAILER-DAEMON: nobody
______________________________________________________________________
This example will forward local mail for the root, news, postmaster,
mail, and www users to fred, while messages for nobody and MAILER-
DAEMON will be redirected to /dev/null.
Just like the genericstable, aliases may contain lots of entries.
Thus, it would once again be inefficient for sendmail to use the text
file we just described. The same mechanism as with genericstable is
used for aliases: A hashed database is generated. Instead of using
makemap directly, you can type in the command newaliases this time.
It will automatically take care of all what's needed.
4. Further reading
The sendmail source distribution includes quite a bit of
documentation. Read it, especially the file cf/README.
If you are interested to dive deeper into sendmail's configuration
options, you want to get the "Bat Book" from O'Reilly: Bryan Costales,
Eric Allman, and Neil Rickert: "sendmail". O'Reilly, 1993.